Understanding Mazda's Rotary Engine

As a devout “rotor head” I get it all the time; people coming up to my 2004 Mazda RX-8 to slang some car lingo. Just the other day, my loquacious buddy Carl saw me leaving work. He came up before I had the chance to zoom-zoom, and he marveled at the new sled. He was intrigued by the sleek suicide doors, and remarked about the mix of sharp and rounded lines which make the car look like it is doing triple digits even while standing still (those weren’t his exact words, but I am paraphrasing here, okay?). Then the Tim Taylor style discourse commenced.

“Yeah, it’s got a Wankel Rotary in it,” I boasted.
“It’s a rotary, huh? So what is that, like a V6 or something?”
“Uh, no. It’s not a V6.”
“Well, how many cylinders does it have? Is it like a flat four or an inline?”
“No man, it doesn’t have cylinders. It has two rotors.”
“So it only has two of them? Well that’s pretty weak.”

I popped the hood, and the acronyms soon spewed forth.

“Does it have DOHC?”
“No,” I respond. “It doesn’t have cams, it has ports.”
“Ports? So it isn’t DOHC? Is it like 24V?”
“No, uh, nevermind.”

Just when I though it couldn’t get worse, the two most clichéd questions in automotive dialect were muttered.

“Well, how much horsepower?”
“Uh, I think about 240.”
“What’s it do zero to sixty in?”
“Uh, I don’t remember.” By this point I was beyond perturbed and gave up.
“Hmm, seems pretty weak. You know, the Z has like three hundred horsepower.”

Maybe it is partly my fault for conversing with such an inadequate car aficionado, but I often notice that some aspiring car buffs don’t truly understand what a rotary engine is. For those of you who privately admit to guilt in the aforementioned scenario, this article will hopefully bring an end to the conceptual mystery of the Wankel Rotary Engine.

Felix Wankel – The Father of Rotary Engines

Born and raised in Germany as a mechanical engineer in the first half of the 20^th Century, Felix Wankel was on a mission to create a lighter and more energy efficient combustion engine. Instead of utilizing the traditional reciprocating cylindrical piston, by 1957, Felix developed an engine that exploited a triangular shaped piston (rotor) fit inside an oval-shaped chamber. The rotary design that Wankel engineered was far more efficient because the rotors could spin in a circular motion which maintained its rotational inertia. Wankel’s rotary engine eliminated the inefficient process of vertical piston acceleration and deceleration. The Laws of Physics play to the advantage of the rotary design, which rewarded Felix with a far more energy-efficient combustion engine blueprint.

Advantages of the Wankel Rotary Engine

Since 1957, numerous advances in Wankel’s design have been made, primarily by its biggest proponent, Mazda. After partnering with Wankel, Mazda continued to hone its natural advantages of being lighter weight, smaller in size, and more compact as compared to traditional piston engines. With the absence of a crankshaft, camshaft, valves and valve-train, the Wankel Rotary Engine has close to 30 percent fewer moving parts, which equates to easier maintenance. The rotary engine also boasts more horsepower per liter than any piston engine, with the new Mazda RENESIS producing 186 HP per liter!

The rotary design is also very modular, in that you could potentially bolt one rotor behind another behind another just like a stack of pancakes. Most rotary powered Mazdas that have been sold in the U.S. have been the 1.3 liter two rotor configuration (650cc per rotor). However, in Japan and Australia, Mazda has sold a three rotor Cosmo, and Mazda won the 1991 24 Hours of Lemans in a four rotor, 700 horsepower 787B, the only Japanese manufacturer to ever win that grueling race – a testament to the long racing heritage, reliability, and durability of a Wankel Rotary Engine.

Driving a Rotary

If you’ve ever driven a RX labeled Mazda, such as an RX-2, RX-3, the legendary RX-7, or even the new RX-8, the feeling is unlike any other car. The instant you dig your rubber sole into the accelerator, the hum of Wankel’s baby is unmistakable. Unless you have the twin-turbocharged third generation RX-7 or a three rotor Cosmo, you won’t win any quarter-mile street sprints with a rotary, as its torque at low RPMs is anemic. However, get the tachometer above 5,000 RPMs, and suddenly you enter the realm of Wankeldom; immediate power on demand, high pitched melodic acceleration, and a stratospheric redline.

Redline in most rotary engines doesn’t arrive until a shift buzzer kindly indicates 9,000RPMs, and if daring, it can even be pushed a bit beyond. While most piston engines falter, cough, spit, choke, and moan around redline, a rotary engine is in its element. The power band from 5000 RPMs up to redline is the most linear you will ever experience, and the sound, oh the sound it makes – you may think you are on a Suzuki crotch rocket.

Rotary Weaknesses

So if Wankel’s brainchild was such a groundbreaking automotive discovery and pleasure to drive, why is it that Mazda seems to be the only major global manufacturer still utilizing this engine? The first and most challenging obstacle for the rotary has been its poor fuel efficiency and high emissions. Since the first rotary powered Cosmo rolled off the assembly line in 1967, Mazda has struggled to keep the rotary engine within US EPA emissions standards, and it hasn’t been easy.

Fuel efficiency downright stinks, no doubt about it. For a 1.3 liter engine to get low 20MPG in the highway is pretty pathetic, and don’t even mention what kind of fuel economy one gets when accelerating above 7,000 RPMs. Come to think of it, maybe that sweet sound I hear is actually the sound of gasoline being sucked through the fuel pump.

Manufacturing costs for the rotary are also significantly higher than a comparable piston engine, and in today’s world of ever tightening profit margins, manufacturers can no longer afford to have image defining models like a marquee low-profit sports car.

Another challenge is that in a normally aspirated two rotor configuration, the tiny 1.3 liter engine does not produce enough torque for America’s rare red meat, bigger is better, stump pulling torque, Detroit muscle car roots. Believe it or not, in the early 1970s, GM was only months away from producing a Corvette powered by a rotary engine – then the fuel crisis struck and killed all plans.

The other challenge with rotary engines is they can be somewhat fickle. Because the motor is designed to burn oil, you must constantly keep an eye on oil level. Rotaries also run extremely rich when cold, so all “rotor heads” know that shutting off the car when cold will lead to flooding and fouled sparkplugs. Therefore, a simple process like moving your car 20 feet turns into a 5 minute drive around the block exercise. To the car aficionado, these may be small inconveniences for the uniqueness of owing a rotary powered car, but for the majority of consumers, it is enough to keep them away.

The Future of Rotary Engines

Despite rising fuel costs, and the trendy push to buy hybrid fuel efficient cars, Mazda’s RX-8 has done surprisingly well since its debut in July of 2003. Since that time, sales of the RX-8 have met or exceeded Mazda’s expectations, which is a big relief for those who wish to see the rotary dream kept alive.

What does the future hold for the engine that Felix Wankel originally developed nearly 50 years ago? Although the rotary engine is less fuel efficient than traditional piston engines, the new Mazda RENESIS rotary is far more efficient in a hydrogen fuel burning application. Because the motor utilizes separate chambers for intake and combustion – which is more ideal for burning hydrogen without backfiring – Mazda engineers have been able to develop a hybrid gasoline and hydrogen powered RX-8, which is currently being road tested in Japan.

This development may breathe new life into the viability of the rotary engine. With increasing attention being given to alternative fuels, the Mazda RENESIS rotary may help lead other manufacturers into the next generation of energy efficient vehicles, and keep the rotary engine off of the automotive endangered species list forever.